Page 94 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

are familiar with flyback direct-current-direct-current (DC-DC) switching converters

would know this feature.
In addition, using inductive or transformer loads in class A power amplifiers allows
for a theoretical efficiency of up to 50 percent. In contrast, a resistive-load class A
power amplifier yields at most about 25 percent efficiency.
In the follow-up to the one-transistor reflex radio, the multiple-transistor reflex

radio, a second amplifier will be added for driving low-iimpedance headphones or
speakers and will use an output transformer.
So now let's take a look at the one-transistor reflex radio in Figure 6-3.
Parts List

• Cl, C7: 0.01 ~F
• C2, C6: 1 ~F, 35 volts

• C3: 0.0022 IJF
• C4: 100 IJF, 16 volts
• R1: 47 k

• R2, R4: 1 k

• R3, R5: 100 k

• R6: 200 k

• T1 audio transfomer: 10 k

primary, 10 k

or 2 k

secondary

• L1 antenna coil: 470 IJH primary, 23 IJH secondary
• L2: 3.9 mH or 4.7 mH
• VCl two-section variable capacitor: 140 pF, 60 pF

• 01, 02: lN914
• 03: lN34
• Ql: MPSH10
In this particular design, a 680-IJH ferrite antenna coil was used. It has a tap at 470

IJH that matches a variable capacitor of about 200 pF. Thus both sections of a
variable capacitor with 140 pF and 60 pF work fine with the primary winding of Ll.